Atrophy of skeletal muscle is part of the aging process and leads to significant morbidity and mortality in the elderly population. Although the anatomical and physiological changes of aging muscle have been well described, the cellular and molecular bases of those changes remain elusive. The primary hypothesis of this proposal, based on the free radical theory of aging, is that oxidative injury leads to age-related muscle atrophy. We will test this hypothesis in mouse, since rodents have been shown to be good models of human muscle aging, and because using mouse models provides two distinct advantages: genetic analysis and in vitro analysis. As such, the Specific Aims of this proposal are: 1) To test the role of oxidative stress in age-related muscle atrophy;: biochemical and genetic studies; and 2) To study the susceptibility of young and old muscle to oxidative injury: in vitro studies. For Aim 1, we will analyze specific muscles of adult (9 mos) and old (24 mos) mice to obtain quantitative histologic assessments of age-related muscle atrophy. We will test the free radical hypothesis by examining the muscles for biochemical and molecular changes indicative of oxidative stress: lipid peroxidation will be measured as a direct indication of oxidative damage, and induction of antioxidant genes will be assessed as an indirect measure of oxidative stress. For the genetic studies, we will perform identical analyses in strains that have genetic variations in free radical metabolic pathways. We will focus on two strains that we believe may be most informative: 1) a transgenic strain that overexpresses the Cu,Zn superoxide dismutase (SOD-1) gene, since SOD overexpression has been shown to inhibit age-related changes in other species; and 2) a knockout strain that has a deletion in the nitric oxide synthase (NOS) gene expressed in muscle, since nitric oxide (which is produced by NOS) has been shown to mediate oxidative injury and muscle wasting. For Aim 2, we will use differentiated cultures of muscle cells isolated from young and old mice. Myotube cultures will be compared in assays of cell injury to determine the relative susceptibilities to oxidative injury as a test of the free radical hypothesis. In addition, oxidative injury and antioxidant capacities will be compared in young and old myotube cultures. These multifaceted studies should provide a broad test of the free radical hypothesis to investigate basic molecular and cellular mechanisms underlying age- related muscle atrophy.